Discharge lamps need to be ignited by applying a voltage higher than a threshold voltage, indicated as breakthrough voltage. For sustaining the discharge, a lower voltage suffices. The level of the breakthrough voltage depends on certain conditions, such as the temperature of the lamp electrodes: at higher temperature, the breakthrough voltage is lower. For generating an ignition voltage pulse, it is known to arrange a ballast comprising an inductor in series with the lamp and to arrange a switch in parallel with the lamp. In a first step, the switch is closed (i.e. conductive), so that a current flows through the inductor and the lamp electrodes to heat the electrodes and to charge the inductor. In a second step, the switch is opened (i.e. made non-conductive), so that the inductor generates a high-voltage pulse over the lamp electrodes. This high-voltage pulse causes a discharge in the lamp, which is accompanied by a visible light flash. If the conductivity in the discharge channel is sufficient, the discharge can be sustained by the mains voltage. In practice, it may take a few of these switching cycles before the lamp ignites.
A conventional example of such starter switch is a glow switch. A glow switch basically consists of bimetal contacts which in normal conditions are opened. If a voltage is applied, a small glow current flows through the switch, the glow discharge heating the contacts so that they close. In the closed condition, the switch carries a larger current to heat the lamp electrodes, but the bimetal switch contacts cool down so that after a few seconds they open again.
At the end of the operational lifetime of the lamp, ignition becomes more difficult, briefly speaking because the electrodes are exhausted. If no precautionary measures are taken, a normal glow switch starter will keep on switching, resulting in repeated discharge flashes in the lamp, which is visible as a flickering of the lamp. This flickering is experienced by people as irritating. Additionally, as a result of the continuous application of large currents, the starter, the lamp tube and the ballast may become hot, which is potentially dangerous.
This problem is already recognized in the art, and several solutions for solving this problem have already been proposed.
US-2003/0.085.668 discloses the use of a semiconductor switch in combination with a solid state timer which limits the time during which the switch attempts to start the lamp.
GB 2.254.970 discloses a starter device for a fluorescent lamp, comprising a glow igniter, a resettable bimetallic switch, and an NTC resistor connected in series to one another, and further comprising an ohmic resistor arranged in parallel with the NTC resistor. The current through the glow igniter causes heat development in the resistors. This heat is transferred to the bimetallic switch. Normally, the lamp ignites after a few switching cycles, and the starter circuit remains without current as from the moment of ignition so the heat generation is stopped. Normally, the heat developed till this moment is not sufficient to actuate the bimetallic switch. In the event of the lamp failing to ignite, the continued heat generated in the resistors causes the temperature of the bimetallic switch to rise sufficiently so that it opens, thereby interrupting the electrode heating circuit. In this case, the bimetallic switch is of a type which does not close automatically on cooling down: the switch needs to be reset manually by a user. Thus, as long as the user does not reset, the irritating flickering of the lamp is stopped.
The device of GB 2.254.970 has several disadvantages. An important disadvantage is that this device poses a safety risk particularly in an oil, gas or chemical industry environment. When the switch opens, the current is interrupted the hard way by contacts that go apart, and a flashover may occur, which is particularly unsafe in situations with flammable gases. Further, it may happen that the bimetallic switch is blocked, i.e. it does not open; in that case, the ignition process will continue, so that parts of the lamp assembly may get heated, which also may cause unsafe situations.
An object of the present invention is to provide a starter for a gas discharge lamp, which is inherently safe in its operation.